The long-term objective of this proposal is to gain insight into mechanisms of cutaneous host defense against the common bacterial skin pathogen Staphylococcus aureus. S. aureus is responsible for the vast majority of skin and soft tissue infections such as impetigo, folliculitis, and cellulitis. In addition, S. aureus can often spread from the skin and lead to invasive and frequently life-threatening infections such as lymphangitis, septic arthritis, osteomyelitis, bacteremia, pneumonia, abscesses of various organs, meningitis, endocarditis, and sepsis. Despite advances in conventional antibiotic therapy, the incidence of S. aureus infections have continued to increase and many hospital- and community-acquired infections have been complicated by the widespread emergence of antibiotic resistant strains. These strains include methicillin-resistant S. aureus (MRSA), multi-drug resistant strains, and even strains that are resistant to vancomycin, the last drug to which the organism used to be uniformly sensitive. These resistant strains have become a significant public health problem causing not only morbidity but even deaths in previously healthy adult and pediatric patients. In time, without the development of new and effective antibacterial therapies, it is possible that multi-drug resistant S. aureus strains will be untreatable by conventional antibiotics. In the present proposal, we plan to investigate the mechanisms involved in host defense and neutrophil recruitment against S. aureus infections in the skin, where most of these infections originate. The recruitment of neutrophils to the site of infection is the first line of defense against S. aureus infection and is also required for elimination of the pathogen. Our recent findings demonstrate that activation of IL-1R-signaling by resident skin cells is critical for recruitment of neutrophils to a site of S. aureus infection in the skin. Since IL-1R is activated by IL-11 and IL-12, we hypothesize that either or both of these cytokines are important in promoting neutrophil recruitment in vivo. We further hypothesize that activation of Toll like receptors (TLRs) may lead to production of IL-11 and IL-12 and also promote neutrophil recruitment. We propose to investigate the mechanisms that promote IL-1R-mediated neutrophil recruitment by using an in vivo mouse cutaneous infection model and in vitro human organotypic keratinocyte cultures. This proposal should provide important new insights into mechanisms of cutaneous host defense against bacterial pathogens. We believe that this study is timely and relevant since the incidence of S. aureus infections is increasing and the treatment of these infections has become exceedingly difficult due to the emergence of antibiotic resistant strains. RELEVANCE TO PUBLIC HEALTH Staphylococcus aureus skin infections are a significant public health problem resulting in 11.6 million outpatient and emergency room visits and 464,000 hospital admissions per year in the U.S. In the present proposal, we plan to investigate mechanisms of neutrophil recruitment against S. aureus skin infection in the skin, where most of S. aureus infections originate. We believe that this proposal is timely and relevant, because the incidence of S. aureus skin infections is increasing and hospital- and community-acquired infections caused by methicillin-resistant S. aureus (MRSA) and multi-drug resistant strains have emerged as major public health threats in the United States.